FIGS. 1 and 2 are diagrams showing the construction of conventional Multiple-Input Multiple-Output (MIMO) antennas. Each of a plurality of antenna elements 10 that constitutes a conventional MIMO antenna includes a radiator 11 and a feed point 12, and is connected to a ground surface 13. Since a conventional MIMO antenna, in which a plurality of antenna elements are arranged and which performs multiple input/output operations, is mounted in a small-sized mobile communication terminal, the distance between the antenna elements must be short, in which case electromagnetic waves radiated from the antenna elements cause mutual interference. The conventional MIMO antennas that have been devised to overcome this problem are designed to improve the degree of isolation. This has been done by ensuring there is sufficient distance between the feed points 12 of the antenna elements 10, as shown in FIG. 1, or alternatively, by forming slits 14 corresponding to 0.25λ of a frequency band for which the degree of insulation is desired to be improved in the ground surface 20 to which the antenna elements 10 are connected, as shown in FIG. 2. The results are that the flow of current components is directed to the slits 14 formed in the portion of the ground surface 13 below the space between the antenna elements 10, thereby reducing mutual interference of electromagnetic waves.
However, since the technology used to construct the above-described conventional MIMO antenna reduces the degree of insulation if a sufficient distance is not ensured, unlike that of FIG. 1, a distance equal to or longer than a predetermined distance must always be secured. Currently, the appropriate distance between the antenna elements 10 of a typical MIMO antenna is equal to or longer than 0.5λ.
Furthermore, in the case where in order to overcome the problem of the antenna of FIG. 1, the slots 14 are formed in the ground surface 13, as shown in FIG. 2, it is difficult to mount part of another element in the area of the ground surface 13 where the slots 14 are formed. Also, the location where part of another element can be mounted cannot be freely selected, so there are problems in that circuit configuration and design implementation are limited and are not flexible.